Dispersion Angle for rapid fire WW2 aircraft guns

[beet1e]

In a recent discussion in the GD forum, Lazs and I were debating the viability of long range gunnery. We all know that WW2 aircraft guns had bullet dispersion built into them to give the pilots a better chance of hitting the target, but my question is: What was the dispersion angle of various weapons, eg. .50 cal, cannon, .303 etc. Was it 1°? 5°? 10°? I would have thought it was the low end of that range, but I really don't know.

I tried a google search for a discussion about this, and it turned up this thread on this very board from a few years ago! Quite a good thread really, and worth another look for those interested.

[Charge]

"We all know that WW2 aircraft guns had bullet dispersion built into them to give the pilots a better chance of hitting the target"

I didn't know that. I thought it was mostly a negative side effect of high MV and high ROF usually accompanied by a too flexible mounting of the gun (eg. wing).

-C+

[Grendel]

Plus heating of gun barrels, again increasing dispersion.

No, there was so such thing as "built in dispersion". The idea was to have the gun shoot its bullets in as tight pattern as possible.

In reality the factors mentioned above and others caused dispersion.

The guns could be of course set to shoot different patterns, that would give lessed skilled pilots larger effective area - but with less bullets hitting the target.

[gripen]

Originally posted by Grendel

No, there was so such thing as "built in dispersion". The idea was to have the gun shoot its bullets in as tight pattern as possible.

Hm... I can certainly say that some guns have purpose built dispersion in mountings, and some of these are used still used today with notable succes (like the GSh-23). I don't know the excat value of the dispersion, but around couple degrees.

In ideal conditions (assuming that aiming was perfect and not systematically off) dispersion is bad. But in reality during WWII aiming was nearly allways systematically wrong because there was no way to determine correct lead until early gyroscopic sights arrived (and these still required some pure aproximations for range measurement). Basicly most pilots used too little lead and aiming point was behind and below correct point specially at long range. Therefore some amount of dispersion actually improved probability of the hit and also percentage of the hits.

It should be noted also that aiming error was generally much larger (say 10-30x)  than dispersion at longer ranges.

gripen

[beet1e]

Originally posted by Grendel
No, there was so such thing as "built in dispersion". The idea was to have the gun shoot its bullets in as tight pattern as possible.

Ah rgr that. An army guy I used to know told me about how a belt fed machine gun was set up on the ground to provide cover for the guys moving behind it. Initially, it discharged over too small an area, so the rifling was adjusted (bored out?) to give the gun less accuracy but distribute the bullets over a larger area, which is what was needed in this case. I assumed that they doctored aircraft guns the same way - I guess they'd have to if the guns were naturally as accurate as laser beams.

Gripen! - you wrote a lot in that thread I linked. So - a couple of degrees total spread, ie. 1° in any direction away from dead centre?

[gripen]

beet1e,
Couple degrees is just a wild quess, in fact it might have been less. In the case of the ZU-23 (AA gun, not GSh-23, I forget the designation), the mounting is built to have something like couple mm loosenes in the support point. Notable thing is that support point is actually rectangular so I quess the pattern is also rectangular. While shooting the jumping of the gun in the support point is quite well visible as well as dispersion of the projectiles.

gripen

[Oldman731]

Originally posted by Grendel
No, there was so such thing as "built in dispersion". The idea was to have the gun shoot its bullets in as tight pattern as possible.

I think Beetle is referring to the practice followed by the RAF at the beginning of the war to deliberately arrange their guns to fire in a circular pattern, similar to a shotgun pattern.  It was called the "Dowding Spread," if I recall correctly.  The idea was that pilots wouldn't be able to shoot an accurate stream of bullets and hit a fast enemy plane.
Each of the eight guns was therefore aimed a slightly different direction from the others.

- oldman

[Treize69]

I think I recall reading about either 8th or 9th AAF P-47 pilots doing pretty much the same thing (spreading the bullets) when they were flying predominantly ground-attack missions in the summer and fall of 1944. I forget the specifics, but I'm assuming it was similar in concept and application to the aforementioned "Dowding Spread".

They wanted to spread the destruction at ground level over as wide an area as possible without having to make multiple runs at the target area.

[gripen]

Originally posted by Oldman731
 The idea was that pilots wouldn't be able to shoot an accurate stream of bullets and hit a fast enemy plane.
Each of the eight guns was therefore aimed a slightly different direction from the others.

That's infact a different way to reach same advantage as built in dispersion in the mounting of a single gun but the result is almost same except the pattern would be slightly different (wider).

gripen

[hitech]

beetle: Typicly less than 1 deg.

To visulize what 1 deg is.

The sun and moon are both 0.5 degs across.


HiTech

[HoHun]

Hi Beet1e,

>In a recent discussion in the GD forum, Lazs and I were debating the viability of long range gunnery.

It seems that the term "effective range" is lacking a clear definition in air combat. Accordingly, it's not suprising there is so much contradictory material around.

>We all know that WW2 aircraft guns had bullet dispersion built into them to give the pilots a better chance of hitting the target, but my question is: What was the dispersion angle of various weapons, eg. .50 cal, cannon, .303 etc. Was it 1°? 5°? 10°? I would have thought it was the low end of that range, but I really don't know.

I guess you're going to enjoy this thread:

http://www.hitechcreations.com/forums/showthread.php?s=&threadid=131669

The answers:

"Built-in" dispersion does not make much sense, though it was used initially in WW2. The RAF at some point appears to have adjusted the Spitfire's cannon so that their lines of fire crossed the centreline at different points in the vertical, which could be understood as a way of creating one-dimensional dispersion. Most harmonization patterns, however, were simply set for the greatest density of fire at the most common engagement range.

Here is a link to a highly interesting Luftwaffe gunnery training booklet:

http://rafiger.de/Homepage/Pages/Schiessfibel.html

Page 14 re-inforces I point I've just made: Dispersion doesn't help you. The Schießfibel:

"Accordingly [referring to example illustrations for faulty deflection shooting], don't rely on weapon dispersion - it won't help you if your aim is flawed! You can see here clearly how ACCURATELY you have to know and to apply deflection, or your fire will miss. But if you think now that you could simply adjust your MGs for a larger pattern to hit more reliably, then you're making a mistake. Your experience will be similar to that of the wild hunter in the picture on the right."

The latter advice is repeated on page 28:

"Another thing: Please don't invent new harmonization patterns, for example by adjusting the trajectory cross-over 100 m out etc. The ordered harmonization has been carefully devised by combat veterans and is good."

Here are dispersion values for different types of guns, partly from historical data and partly calculated. These values assume a rigid mount, such as in the aircraft nose - wing mounts usually are less rigid and lead to a higher dispersion. Flexible mounts are just that ;-) so defensive weapons have a much larger dispersion. (There are large differences depending on the mounting - I believe in this regard the Sperry ball turrets were the best of the US turrets.)

The units are mils, with 1 mil = 1 m error at 1000 m range

MG-FF:             1,0
20mm Type 99-1:    1,0
MK 108:            1,5
20mm Type 99-2:    1,5
Ho-1 / Ho-2:       1,5
12,7mm Scotti:     1,6
Breda-SAFAT:       1,7
MG 131:            1,7
20mm Ho-5:         1,9
MG 151/20:         1,9
MK 103:            2,0
20mm ShVAK:        2,0
Hispano V:         2,1
37mm M4:           2,1
Ho-103:            2,2
VYa-23:            2,5
12,7mm UB:         2,7
Hispano II:        3,0
NS-37:             3,3
MG 151:            3,4
,50 Browning M2:   4,0
Browning ,303:     4,2

With regard to long range fire, it's a combination of three factors:

- Trajectory
- Dispersion
- Divergence

(The latter simply describes what happens if you pass the convergence point - the lines of fire diverge.)



Regards,

Henning (HoHun)

[straffo]

Hohun do you have a link or data for  the 190A8 convergence ?
I've lost the drawing somewhere and I'm unable to put my hand on it.

[HoHun]

Hi Straffo,

>Hohun do you have a link or data for  the 190A8 convergence ?

Quick summary:

MG 131 (cowl): max 57 cm above the sight line at 220 m range, sight line crossed at 30 m and 400 m. No convergence (parallel).

MG 151 (wing roots): max 82 cm above the sight line at 360 m range, sight line crossed at 120 m and 550 m. Convergence 600 m.

MG 151 (wings): max 84 cm above the sight line at 360 m range, right line crossed at 135 m and 550 m. Convergence 400 m.

Regards,

Henning (HoHun)

[straffo]

Merci beaucoup

[gripen]

Originally posted by HoHun


I guess you're going to enjoy this thread:

http://www.hitechcreations.com/forums/showthread.php?s=&threadid=131669

Well, let's take a quote from HoHun's post in that thread:

"If you consider the aiming errors random, there's no total advantage for the larger dispersion weapon at all because random errors are centered at the correct aiming point, and large dispersion gives you a big disadvantage there."

The problem is that there is no reason why random errors should be centered at the correct aiming point at long range. The error is probably systematical in most cases because there is no way to determine correct aiming point at long range. Therefore some amount of dispersion gives an advantage.

gripen